1. Field of the Invention
The present invention relates to methods of coating elongated filaments, such as wire, and, more particularly, to such a method that minimizes internal residual stress in the coating.
2. Description of Related Art
Elongated filaments, such as wire, are usually coated with material to protect the elongated filaments from the effects of deleterious environments, and/or to electrically insulate such elongated filaments. A problem encountered with certain coatings is that the coating process introduces stress into the coating material. This internal residual stress can cause undesired failure of the coating when the elongated filament is subjected to bending or torsional forces. Applied bending or twisting motion combined with the coating's internal residual stress causes it to become brittle, crack and peel. If the internal residual stress in the coating material could be minimized, the amount of applied stress needed to cause failure of the coating could be proportionally increased, which will result in a coating that is superior to previous coatings.
Illustrative methods of coating elongated filaments are disclosed in U.S. Pat. Nos. 4,391,848; 4,393,809; 4,394,417; and 4,489,130; however the issue of minimizing internal residual stress is not addressed therein.
Internal residual stress is caused when a plastic or polymerized material cools, in that the plastic cools from the outside inwardly and shrinks. This shrinkage, which is analogous to an increase in density, generates internal stresses in the plastic. Plastic coatings on wire have a special problem in that as the outer surface cools and solidifies, the still molten plastic (beneath a thin skin of cooled plastic) must shrink as it cools, and at the same time occupy the same volume because the outer skin is now a solid. Thus, this internal residual stress will result (on a microscopic level) in voids between the wire and the coating or within the coating itself. Polymers that tend to crystallize, such as polyetherketone (PEK) and polyetheretherketone (PEEK), exhibit this problem to a greater extent than noncrystalline coatings. Crystallization is a more ordered packing of the polymer chains, and consequently crystallized polymer coating occupy less volume and have a higher density than the noncrystallized portions of the polymer coating. Thus, a significant density gradient occurs at the interface of the microscopic crystals and the noncrystallized polymer coating.
An example of the above described problem of internal residual stress occurs with magnet wire used in the windings of electrical motors. Magnet wire is usually coated with a plastic coating material, such as PEK or PEEK, and is tightly wrapped as windings within the electric motor. Unfortunately, in the winding process the coating on the magnet wire can be mechanically damaged by improper bending and/or abrasion. Also, the magnet wires tend to rub against each other as the electric motor vibrates and the electric motor's internal temperature increases and decreases during operation. After many months of this rubbing, it has been found that the coating on the magnet wire becomes brittle, and minute cracks usually result. These cracks permit environmental agents to attack the now unprotected wire conductor causing hot spots, corrosion, and most troublesome, electrical shorting.
A more specific example of this problem is found when this prior type of magnet wire is used in an electric motor that powers an electric submergible pump (ESP) in a well. Such ESP's must operate without failure for years while submerged in wellbore fluids containing hydrogen sulfide and other chemical agents, as well as at temperatures oftentimes above 200 degrees F. When the coating on the magnet wire fails, as described above, the electric motor usually shorts and fails, the production of fluids from the well ceases, and a workover crew and equipment must remove the failed ESP and reinstall a replacement ESP. This failure results in lost oil production, which costs many thousands of dollars in lost revenue, and unnecessary associated ESP replacement costs.
There is a need for a method of coating elongated filaments with a coating that has superior characteristics to those obtainable using prior methods.